Electric contact



Patented Sept. 28, 1937 PATENT OFFICE EIECTBIO CONTACT Phillip n. Estes,Rutherford. N. 1., and Frances H. Clark, New York, N. Y2, assignors toThe Western Union Telegraph Company,

New

York, N. Y., a corporation of New York No Drawing. Application Julyl,1936,

Serial No. 88,496 i Claims.

The present invention relates to circuit making and breaking electrodesfor electrical apparatus and more particularly to improved metalliccompositions or alloys for use in making such 5 electrodes.

The primary object of the invention is to provide circuit making andbreaking contacts having improved characteristics and long life inservice.

Contacts formed of either tungsten or plati- 19 num alone when operatingin service which requires 1,000,000 contact operating cycles in 24 hoursmust receive frequent maintenance and in fact will not operate for 24hours without attention. Very hard alloy contacts, for example 5 thosemade of tungsten carbide combined with cobalt and tungsten carbidecombined with platinum group metals require attention about every 30days in the service just mentioned.

A further object of the invention therefore is 20 to provide anelectrical contact which will operate in high speed service for longperiods of time without requiring attention. Contacts of the presentinvention will operate in service requiring 1,000,000 contact operatingcycles in 24 25 hours for 8 months or over without attention. Electricalcontacts of the prior art formed of tungsten alloys are subject tolocalized accretion of material on one contact, reducing the operatinggap and in time preventing separation of the contacts.

Accordingly, another important object of the invention is to provide atungsten alloy contactwhich is free from any tendency to transfermaterial from its contact surface to another coopcrating contactsurface.

More specifically, objects of the invention are to provide electricalcontacts having relatively low electrical resistance, a high degree ofhardness to resist mechanical wear, a high melting point to preventfusion from service arcing possible under certain conditions of use andcombining also to a degree not heretofore obtained in alloy contacts,the ability to preserve the original contact surface contour.

Tungsten alloy contacts have been made of a sintered alloy comprisingtungsten initially in a granular or powdered condition and a metal'fromthe platinum group, also initially in a granular or powdered condition.In prior art tungsten alloy 50 contacts made up in the manner juststated,

transfer of metal from one contact to another in service is caused bydeterioration at the boundaries of the minute grains of tungsten. Thisdeterioration in time permits the grains to be sepa- 55 rated withcomparative case from the main body of the contact. This separation, wehave discovered, is caused by either one of two conditions. The firstcondition is an insumcient alloying of the added metal with the tungstenso that some of the grain boundaries are imperfectly coated. The secondcondition is an excess of the added metal which may actually lie betweenthe grains so as to physically separate them. Also, where too much addedmetal is present, it occupies portions of the working face of the con-10 tact, and these portions however minute, are transferred to theworking face of a cooperating contact.

By the present invention the tendency toward accretion is substantiallyeliminated by using 15 only such an amount of platinum group metal thatall of the tungsten grain boundaries throughout the contact will becovered with a solution comprising tungsten and the added platinum groupmetal without leaving an uncomblned excess of added metal.

The discovery that for feeble currents of the order of one quarter of anampere or less, contacts are most suitable when the tungsten has anexceedingly small grain size is a further im- 25 portant feature of theinvention. The preferred size of tungsten grain for contacts to .be usedfor low current is approximately. .00015 inch. If the size of theindividual tungsten grains in the contact material is larger than .0004inch, the 30 contact is susceptible to excessive erosion. Grain sizes inthe upper range between these two limits may also be suitable forcontacts carrying heavy currents but are not as satisfactory for lightcurrent operation whereas the operating speeds 35 \are high. Contactssuitable for larger currents have a grain size between the firstmentioned preferred size and the maximum size.

It is essential in producing contacts in accordance with the presentinvention, that each grain or minute particle of tungsten present asurface which consists only of a solution of the added noble metal andthe tungsten. By this grain structure intimate and perfect cohesion isse- 45 cured between the grains or particles of the contact, and thecontact presents a relatively uni-- form working surface.

In carrying out the invention, contact material is formed from tungstenand from 2 to 8% by weight of a noble metal of" the platinum group. Bythe term metal of the platinum group is meant rhodium, ruthenium,platinum, osmium and iridium which are the preferred added metals to beused in contacts embodying the invention. The materials employed consistof finely divided particles of substantially pure tungsten and therequired amount of the selected platinum metal also in a finely dividedform. 5 The comminuted metals are prepared in accordance with well knownpractices, preferably by powdering in a ball mill or equivalentapparatus for approximately seventy-two hours, after which the powder ispassed through a fine mesh screen having openings approximately .0017inch in size. The powdered materials of which the contact is to be madeare thoroughly intermixed in a ball mill for approximately 24 hours.

The intermixed metals are molded to a desired form, usually that of afinished contact, under a pressure of from 30,000 to 120,000 lbs. per

square inch. The molded contacts are then sintered in accordance with awell known practice in an atmosphere of hydrogen at a temperature lessthan 5000 E, which is approximately the melting point of tungsten. Thepreferred sintering temperature is approximately 2900 F. with a pointsomewhat below the melt ing point of tungsten as the upper limit. Forexample, contacts which operate successfully in severe service areproduced in accordance with the present invention by sintering at atemperature of 3000 F., the sintering time being approximately twohours. However, contacts produced by sintering at temperatures of 2800F. and under show signs of early failure. Contacts produced inaccordance with the invention are ready for use following the sinteringoperation as it is not necessary to polish or otherwise prepare them forservice.

The procedure just described is used when producing contacts embodyingany one of the named platinum group metals.

It will be seen from the foregoing that contacts produced in accordancewith the present invention are particularly useful for high speedoperation when handling small currents and that in any service for whichthe contact is 45 designed uniform wear is experienced at the contactsurface without any tendency toward local accretion.

We claim:

1. An electrical contact element consisting of finely divided tungstenand a noble metal from the platinum group, the boundaries of thetungsten particles being composed 01' a solution of tungsten and thenoble metal, the cohesive union between all particles of the contactbeing provided only by said solution whereby cohesion between saidparticles is uniform throughout the contact and on the exposed workingsurface thereof.

2. The contact as defined by claim 1 wherein said noble metal isplatinum comprising from two to eight per cent by weight 01' saidcontact.

3. The contact as defined by claim 1 wherein said noble metal is rhodiumcomprising from two -to eight per cent by weight of said contact.

4. The contact as defined by claim '1 wherein' said noble metal isrhodium.

8. The contact as defined by claim 5 wherein said noble metal is osmium.

9. An electrical contact composed of a homogeneous sintered mass offinely divided tungsten, the said tungsten being combined by solutionwith an entire quantity of added noble metal from the platinum group.

10. An electrical contact composed of a sintered mass of finely dividedtungsten having a grain size of approximately .00015 inch and not over.0004 inch, the said tungsten being combined by solution with auniformly distributed quantity of a noble metal from the platinum group.

PHILLIP H. ESTES. FRANCES H. CLARK.

